As one of the important characteristic parameters of a single-mode fibre, the mode field diameter (MFD), directly influences important characteristics of splicing loss, anti-bending performance and the like of the fibre. It is also a necessarily-measured parameter in production and application of optical fibres and cables. At present, the yield of the single-mode fibre has reached hundreds of millions of core kilometers in the world, and the huge measurement workload can be imagined. To constantly explore a method for rapidly and accurately measuring these data is always a common goal for scientific workers in the industry.
With respect to measurement of the MFD, three existing methods recognized by international standards are as follows: a far field scan method as a reference test method, a variable aperture method as an alternative test method, and a near field scan method. These standard test methods are all based on the Petermann II definition. A primary formula of the definition is a ratio of two integrals. In these standard methods, the integrals are not processed as a whole, but are segmented into a plurality of parts to be sampled; and then the parts are combined together according to a corresponding formula; so as an integral formula in the Petermann II definition is approximated as summation of several finite term series. In such a process, there is obviously a deviation. To reduce this deviation, more items have to be segmented and measured, which inevitably leads to an increase of measurement time (time consumed for measuring a mode field at a wavelength often takes tens of seconds, or even a few minutes), thereby reducing the measurement speed. Such a basic contradiction cannot be solved through the existing standard test methods. A novel way must be explored and a novel measurement method must be searched. Regards to the mask method, its improvement enables measurement of the MFD rapidly and accurately (see reference document 1) and measurement accuracy can reach better than 0.05 μm (see reference document 2) which is as good as that in the variable aperture method, however, theoretical preciseness has been influenced as a sacrificed in the mask method due to a paraxial approximation adopted in geometrical optics.    Reference document 1: Zhou Wenjun, Theoretical and Experimental Study of Mask Measurement System Chinese Journal of Electronics, Volume 17, Issue 6, (November, 1989), Page 61.    Reference document 2: Multi-Parameter Test Apparatus for Single-Mode fibre, Registration No. 902548, Bulletin of Scientific & Technological Achievements, (1992), State Scientific and Technological Commission.